Primary sex determination in birds depends on DMRT1 dosage, but gonadal sex does not determine adult secondary sex characteristics

Abstract

In birds, males are the homogametic sex (ZZ) and females the heterogametic sex (ZW). Primary sex determination is thought to depend on a sex chromosome gene dosage mechanism, and the most likely sex determinant is the Z chromosome gene Doublesex and Mab-3–Related Transcription factor 1 (DMRT1). To clarify this issue, we used a CRISPR-Cas9–based monoallelic targeting approach and sterile surrogate hosts to generate birds with targeted mutations in the DMRT1 gene. The resulting chromosomally male (ZZ) chicken with a single functional copy of DMRT1 developed ovaries in place of testes, demonstrating the avian sex-determining mechanism is based on DMRT1 dosage. These ZZ ovaries expressed typical female markers and showed clear evidence of follicular development. However, these ZZ adult birds with an ovary in place of testes were indistinguishable in appearance to wild-type adult males, supporting the concept of cell-autonomous sex identity (CASI) in birds. In experiments where estrogen synthesis was blocked in control ZW embryos, the resulting gonads developed as testes. In contrast, if estrogen synthesis was blocked in ZW embryos that lacked DMRT1, the gonads invariably adopted an ovarian fate. Our analysis shows that DMRT1 is the key sex determination switch in birds and that it is essential for testis development, but that production of estrogen is also a key factor in primary sex determination in chickens, and that this production is linked to DMRT1 expression.

Sex chromosome evolution among amniotes: is the origin of sex chromosomes non-random?

“…For the more robust test of the hypothesis that syntenic blocks with genes with higher potential to become sex-determining genes have a higher chance to become sex chromosomes, we will need a better knowledge on the identity of sex-determining genes in amniotes. Up to date knowledge on sex-determining genes is limited to viviparous mammals [117] and to strongly supported candidates in birds and monotremes [21,118,119].…”

Expanding the classical paradigm: what we have learnt from vertebrates about sex chromosome evolution

“…Potentially, GRC is preferentially segregated to eggs instead of polar bodies in female meiosis (meiotic drive), and it is a functional element of songbird germline genomes [96]. As far as is known, all birds have conserved ZZ/ZW sex chromosomes [97], and genes linked to the avian sex chromosomes contribute largely to cell-autonomous sexual differences in somatic tissues, including gonadal somatic cells in the medulla, as demonstrated in chicken, a nonpasserine bird [98][99][100]. Chicken gonads (both ovaries and testes) can differentiate in the absence of germ cells [101].…”

A brief review of vertebrate sex evolution with a pledge for integrative research: towards ‘sexomics’

“…Such a chromosomally male (ZZ) chicken with a single functional copy of Dmrt1 developed ovaries with typical female markers and exhibited follicular development. Interestingly, these animals were indistinguishable in external appearance from wild-type adult males, supporting that the development of male secondary sexual characters is driven by cell-autonomous sex identity and independent of gonadal hormones [272,273]. The rarity of Z0 and ZZW individuals in birds may suggest that these genotypes are often lethal or infertile [274], and that a locus on the W might control dosage compensation of some Z-linked genes [275,276].…”

“…The gene Dmrt1 resides in the oldest evolutionary stratum of the Z-chromosome [268], shared by palaeognath and neognath birds [269–271]. A recent study using a CRISPR-Cas9 based mono-allelic targeting approach with sterile surrogate chicken hosts supports this hypothesis [272]. Such a chromosomally male (ZZ) chicken with a single functional copy of Dmrt1 developed ovaries with typical female markers and exhibited follicular development.…”